Method of and materials for coating stainless steel and article produced thereby



2,8 l 3,8 l 6 Patented Nov. 19, 1 957 METHOD OF AND lVlATERIALS FOR COATING STAINLESS STEEL AND ARTICLE PRODUCED THEREBY George Qtto, Oreland, Pa., assignor to American Chemical Pamt Company, Ambler, Pa., a corporation of Delaware No Drawing. Application May 16, 1955, Serial No. 508,771

17 Claims. (Cl. 148--6.24)

This invention relates to the art of coating steel and is particularly concerned with a novel method and new materials for producing coatings on stainless steel or heat resistant steel alloys as well as to the finished article resulting therefrom.

In the present disclosure the expression stainless steel has its customary meaning, i. e., it refers to steels which are resistant to tarnish, which steels are usually characterized in that they contain elements such as chromium either with or without nickel, cobalt and molybdenum. As is Well known to those skilled in this art, steels of this kind are particularly difficult to work especially in cold drawing through dies.

The present invention is especially designed as an aid in facilitating drawing operations on stainless steel blanks or draw pieces and the principal object of the invention is to provide a process and materials for producing adherent coatings on stainless steel by means of acidulated baths, which processes and materials are well adapted for use on an economically feasible basis and in commercial size operations.

Another object is the provision of a novel process by which a chemically produced coating of a complex nature ideally suited as an aid to drawing or other deforming operations, can be provided on stainless steel in a relatively short time and this without danger of pitting the surface during the coating operation which pitting, heretofore, has been somewhat of a problem in prior art practice requiring the presence in the coating bath of halogens such as chlorides and fluorides.

The coating produced by the present invention is very effective in improving the drawing properties of stainless steel, thus facilitating cold drawing or other deforming operations and increasing the life of drawing dies and associated equipment and thereby permitting the making of deeper draws in a single operation than has been possible heretofore.

Other objects of the invention will appear in the following detailed description but it should be noted that this description is for the purpose of exemplification only and not for limitation except as defined by the appended claims.

The present invention is based upon the discovery that dilute mineral acid baths containing manganese oxalate and sulphur compounds, which sulphur compounds con tain oxygen and are capable of yielding sulphur dioxide and sulphur upon decomposition in acid solution, can be employed to produce on stainless steel surfaces, a complex coating which is excellently suited for use in drawing operations.

The acids which are useful in the present invention are those from the class consisting of sulphuric, sulphamic, sulfurous, nitric and phosphoric acids. For reasons which will appear below, we prefer to use either nitric or phosphoric acid. Examples of suitable sulphur compounds which can be employed include thiosulfates, hydrosulfite, thionates and sulfites.

The manner in which the manganese oxalate is introduced into the bath makes little or no difference in attaining the objectives of the invention. For instance, it may be introduced in the form of manganese oxalate itself, or it may be formed in the bath by adding to the acid solution manganese nitrate and oxalic acid, etc.

I have found that the amount of free acidity in the bath should be such that the bath will have a pH of from 0.7 to 1.8 or at the most 2 and preferably between 1.0 and 1.5. If the pH of the bath is below 0.7, the bath becomes too reactive to produce good coatings. If the pH is above 2.0, the reactivity of the bath is too low to produce acceptable coatings.

The amount of manganese oxalate in the bath should be between about 1 and 18 grams per liter and preferably between 10 and 15 grams per liter. The presence of excess oxalic acid, over and above that required to form manganese oxalate has been found to materially increase the adherence of the film. Two mols of oxalic acid for each mol of manganese oxalate appears optimum. More merely loads the bath. If the concentration of the manganese oxalate in the bath is below 1 gram per liter, the film or coating formed on the stainless steel becomes relatively ineffective as an aid to drawing. If the concentration of manganese oxalate exceeds 18 grams per liter, the manganese oxalate will precipitate from the solution, thus creating undesirable sludge and wasting valuable chemical.

In normal operation, the bath is operated at a temperature of between F. and 170 F. Below 120 F. (and the bath may be used below this temperature if desired) in normal commercial operation the time necessary to form the coating on the stainless steel is usually too long to be feasible. Above the upper limit of temperature, the bath is prone to give off sulphur dioxide and hydrogen sulphide fumes in such quantities as to be obnoxious and, therefore, I prefer to operate at temperatures below 170 F. In general I prefer to maintain the bath temperatures at between -160 F.

To prepare a solution for coating stainless steel there may be added to water:

(1) Sufficient manganese ion in the form of manganese phosphate or any manganese salt soluble at a pH of about 1 to give a manganese content of at least 004% (Weight/ volume, grams per 100 ml.). I prefer to use manganese phosphate or manganese nitrate as I have found that the presence of a nitrate or phosphate ion in the solution has no effect on either the stainless steel or the coating. They are also readily available manganese salts.

(2) Sufiicient oxalic acid, or a salt of oxalic acid soluble at a pH of about 1, to give an oxalic acid content of at least 0.3% (weight/volume, grams per 100 ml.). As previously stated, excess of oxalic acid is preferred as the excess also appears to increase the adherence of the chemical film. I prefer to have at least twice as much oxalic acid present as is needed to form manganese oxalate.

(3) Sufficient free mineral acid, or salt of the mineral acid, which will liberate free hydrogen ions to produce a pH of less than 2, but not less than 0.7 and preferably between 1.0 and 1.5. Only sufficient mineral acid should be added to completely dissolve the manganese oxalate formed and to produce the desired pH. As previously stated, useful mineral acids are those from the class consisting of sulfuric, nitric, phosphoric, sulfamic and sulfurous acids and I prefer to use either nitric or phosphoric acid. My preference for nitric or phosphoric acid is based upon the fact that, where the bath is used for processing large quantities of stainless steels having a high chromium content, it is possible, apparently, to form in the bath a chrome-oxalate complex with the other mineral acids mentioned which necessitates the use of excessively large quantities of oxalic acid in order to keep the solutions operatable. For some unknown reason, such complexes do not form readily with either nitric or phosphoric acid.

(4) Sufficient of one or more sulphur-bearing compounds containing oxygen which will liberate sulphur dioxide and sulphur at a pH of about 1, such that a 10 ml sample of a bath will react with at least 4 ml of 0.05 normal iodine solution, using starch indicator, to a blue endpoint. These compounds, as stated above, may be thiosulphates, sulphites, hydrosulphites or thionates. I prefer to use a bisulphite, such as sodium bisulphite, and l have found that an admixture of 1 to 7.0% of sodium, thiosulphate and 80 to 99% of sodium bisulphite produces exceptionally good results.

A typical working solution which has been found to give excellent results is:

Formula #1 Manganese sulphate, 70% grams l5 Oxalic acid, commercial grade do Sulfuric acid, concentrated technical grade ml 5 Sodium metabisulphite grams..- 4.5 Sodium thiosulphate do c 0.5

Water, to make 1 liter.

The surface of the piece to be coated should be properly cleaned and with stainless steels of the types which are known to be passive or which readily become passive (i.e.. 18-8 and other high chromium nickel steels particularly when these steels are in the austenitic condition) I have found it highly desirable, in order to obtain an adherent coating in a reasonable time, to depassivate the surface by treating it with an activating acid pickling solution containing hydrofluoric, hydrochloric and/or sulphuric acid as, for example, per one of the baths A. B and C below:

A. Nitric acid (sp. gr. 1.42)--- 8-20 parts by volume.

Hydrofiuoric acid (48% 1-4 parts by volume. Water to make 100 parts by volume. Temperature "JD-150 F.

B. Hydrochloric acid tsp. gr.

1.19) 10-20 parts by volume. Water to make 100 parts by volume. Temperature 70130 F.

C. Sulphuric acid tsp. gr.

1.84) 10 parts by volume. Hydrochloric acij tsp. gr.

L19] 5 parts by volume.

Hydrofluoric acid (48% 0.5 part by volume. Water to make l00 parts by volume. Temperature a. 70-l30 F.

Such activating baths will also serve to remove scale. Where severe scaling requires other types of pickling treatment. a tinal short treatment in a solution such as exemplified not; will assure an adherent coating in a properly maintained coating bath.

The cleaned and activated work is then immersed in the solution of Formula #1 and horn there until a coating h is formed thereon. If the solution of Formula l is heated to approximately l50'l60 F, 3 minutes immersion usually suihccs for the formation of an acceptable coating. After the coating has been formed, the contctl metal ol izct may be withdrawn from the coating solution, i'iietl with water, further lubricated with a cmiveniionil drawing lubricant. dried and drawn.

it ould noted at this time that the coatings formed by the present invention are chcn cully bonded to the writ-cc oi the steel, .tc complex in nature and should not be confined with conventional o alate coatings hertztufor-r ltnovwn as useful in the drawing of :rtainlc steel. Usually thc coating s .ms to be composed of a yellow to olive outer covering with a gray to black colored under coating." Apparently the coatings formed are -duplex in nature in that a portion of the coating seems to serve us a parting material preventing metal to metal ontact, while another portion of the coating seems to solution or are serve as a reservoir for holding a conventional lubricant. 1n the preferred embodiment of the process of the present invention the coatings, after being rinsed with water, are given a subsequent lubrication of grease, soaps or other conventional drawing lubricant.

Other representative bath formulas for use in the present invention are as follows:

Formula #2 Water, to make 1 liter.

Formula #3 Manganese oxalate grams 3 Sulphuric acid rnl 1 Sodium sulphite grams .5

Water. to make 1 liter.

concentration the above bath will time to produce a useable coating,

No'ru: Due to its low require a relatively long even at high temperatures.

Formula #4 Water, to make 1 liter.

It Will be understood, of course, that the foregoing formulas are merely illustrative and that appropriate amounts of sulphamic or sulfurous acids or of mixtures of any of the acids can be employed within the general teachings of this disclosure.

Maintenance of the solution in operating condition during the processing of a succession of pieces requires merely that the proportions of the various active ingredients and the acidity be maintained within the prescribed limits by suitable additions of chemicals. In this connection it will be noted that the coating bath consumes chemicals as follows:

The manganese is mostly removed as a coating of manganese oxalate on the metal. Some appears also to be removed in the coating as a sulphide or other sulphur compound.

The oxalate is partly removed in the coating as manganese oxalate and partly as ferrous oxalate, some of which may be entrapped in the coating but most of which settles from the bath as a sludge.

The sulphur dioxide and associated sulphur compounds are removed from the bath in several ways. The sulphur dioxide appears to be consumed in destroying the passive film on the surface of the stainless steel, thus allowing the tree hydrogen ions to attack and etch or roughen the metal. This removes some of the free hydrogen ions at the solution: metal interface. which in turn locally decreases the solubility of the manganese oxalate, thereby causing a precipitate of manganese oxalate to appear at the interface as an adherent coating. Some volatile sulfur compounds also evaporate from the swept out with evolved hydrogenv The function of the complex compounds of sulphur in the above coating mechanics is not entirely clear, but they appear to accelerate the deposition of manganese oxalate by forming a thin black film of a sulphide of one or more of the heavy metals present.

Hydrogen ion concentration is depleted in the attack on the metal surface and also in reacting with the sulphite ion to liberate sulphur dioxide.

These losses must be replaced in order to keep the composition of the solution within proper operating limits and I find that this can be done by adding thereto from time to time as required manganese nitrate or manganese phosphate and oxalic acid in the following proportions:

30-40% manganese nitrate or manganese phosphate, and 60-70% oxalic acid Additions of these materials in the proportion indicated have been found in most instances to properly replace the manganese, oxalate, and hydrogen ion depletion without the necesstiy of separate analysis for the concentration of each ingredient. In general it has been found preferable to add the sulphur bearing compound separately as it is unstable if mixed with the other replenishing materials.

The present application is a continuation-in-part of my earlier application 472,785, filed December 2, 1954, now abandoned.

I claim:

1. An aqueous, dilute mineral acid metal-coating bath substantially free of halide and consisting essentially of acid from the class consisting of nitric, phosphoric, sulphuric, sulphamic and sulfurous acids; manganese oxalate; and a sulphur-bearing compound from the class which contains oxygen and will yield sulphur dioxide (S02) and sulphur upon decomposition in acid solution; the quantity of manganese oxalate in the bath being between 1 and 18 grams per liter, the quantity of the sulphur compound being such that a ml. sample of the bath will react with at least 4 ml. of 0.5 normal iodine solution, using starch indicator, to a blue endpoint, and the amount of free acidity being such as to yield a pH of from 0.7 to 2.

2. An aqueous, dilute acid metal-coating bath substantially free of halide and consisting essentially of nitric acid, manganese oxalate, and a sulphur-bearing compound from the class which contains oxygen and will yield sulphur dioxide (S02) and sulphur upon decomposition in acid solution, the quantity of manganese oxalate in the bath being between 1 and 18 grams per liter, the quantity of the sulphur compound being such that a 10 ml. sample of the bath will react with at least 4 ml. of 0.05 normal iodine solution, using starch indicator, to a blue endpoint, and the amount of free acidity being such as to yield a pH of from 0.7 to 2.

3. An aqueous, dilute acid metal-coating bath substantially free of halide and consisting essentially of phosphoric acid, manganese oxalate, and a sulphur-bearing compound from the class which contains oxygen and will yield sulphur dioxide (S02) and sulphur upon decomposition in acid solution, the quantity of manganese oxalate in the bath being between 1 and 18 grams per liter, the quantity of the sulphur compound being such that a 10 ml. sample of the bath will react with at least 4 ml. of 0.05 normal iodine solution, using starch indicator, to a blue endpoint, and the amount of free acidity being such as to yield a pH of from 0.7 to 2.

4. The method of coating stainless steel which includes the step of applying a bath according to claim 1 to the surface of the steel at a temperature not exceeding 170 F. until a chemically bonded coating is produced thereon.

5. The method of coating stainless steel which includes the steps of depassivating the surface of the steel and then applying a bath according to claim 1 to the depassivated surface until a chemically bonded coating is produced thereon.

6. The method of coating stainless steel which includes the step of applying the bath of claim 2 to the surface of the steel at a temperature not exceeding F. until a chemically bonded coating is produced thereon.

7. The method of coating stainless steel which includes the step of applying the bath of claim 3 to the surface of the steel at a temperature not exceeding 170 F. until a chemically bonded coating is produced thereon.

8. The method of coating stainless steel which includes the steps of depassivating the surface of the steel and then applying the bath of claim 2 to the depassivated surface until a chemically bonded coating is produced thereon.

9. The method of coating stainless steel which includes the steps of depassivating the surface of the steel and then applying the bath of claim 3 to the depassivated surface until a chemically bonded coating is produced thereon.

10. The method of coating stainless steel which includes the steps of depassivating the surface to the steel by, subjecting it to the action of an acid pickling solution containing acid from the class consisting of hydrofluoric, hydrochloric and sulphuric acids and then applying the bath of claim 2 to the depassivated surface until a chemically bonded coating is produced thereon.

11. The method of coating stainless steel which includes the steps of depassivating the surface of the steel by subjecting it to the action of an acid pickling solution containing acid from the class consisting of hydrofluoric, hydrochloric and sulphuric acids and then applying the bath of claim 3 to the depassivated surface until a chemically bonded coating is produced thereon.

12. A metal coating bath according to claim 1 wherein the quanity of manganese oxalate lies between 10 and 15 grams per liter, the sulphur-bearing compound is chosen from the class which consists of thio-sulfates, hydrosulphite, thionates and sulfites, and the free acidity is such as to yield apH of from 1.0 to 1.5.

13. The method of coating stainless steel which includes the step of applying the bath of claim 12 to the surface of the steel at a temperature not exceeding 170 F. until a chemically bonded coating is produced thereon.

14. A metal coating bath according to claim 2 wherein the quantity of manganese oxalates lies between 10 and 15 grams per liter, the sulphur-bearing compound is chosen from the class which consists of thio-sulfates, hydrosulphite, thionates and sulfites, and the free acidity is such as to yield a pH of from 1.0 to 1.5.

15. A metal coating bath according to claim 3 wherein the quanity of manganese oxalate lies between 10 and 15 grams per liter, the sulphur-bearing compound is chosen from the class which consists of thiosulfates, hydrosulphite, thionates and sulfites, and the free acidity is such as to yield a pH of from 1.0 to 1.5.

16. The method of coating stainless steel which includes the step of applying the bath of claim 14 to the surface of the steel at a temperature not exceeding 170 F. until a chemically bonded coating is produced thereon.

17. The method of coating stainless steel which includes the step of applying the bath of claim 15 to the surface of the steel at a temperature not exceeding 170 F. until a chemically bonded coating is produced thereon.

References Cited in the file of this patent UNITED STATES PATENTS 1,895,568 Curtin Jan. 31, 1933 1,911,537 Tanner May 30, 1933 2,077,450 Weisberg et al. Apr. 20, 1937 2,577,887 Gibson Dec. 11, 1951 FOREIGN PATENTS 683,954 Great Britain Dec. 10, 1952 1,065,191 France Dec. 30, 1953 

1. AN AQUEOUS DILUTE MINERAL ACID METAL-COATING BATH SUBSTANTIALLY FREE OF HALIDE AND CONSISTING ESSENTIALLY OF ACID FROM THE CLASS CONSISTING OF NITRIC, PHOSPHORIC, SULPHURIC, SULPHAMIC AND SULFUROUS ACIDS; MANGANESE OXALATE; AND A SULPHUR-BEARING COMPOUND FROM THE CLASS WHICH CONTAINS OXYGEN AND WILL YIELD SULPHUR DIOXIDE (SO2) ANDND SULPHUR UPON DECOMPOSITION IN ACID SOLUTION; THE QUANTITYY OF MANGANESE OZALATE IN THE BATH BEING BETWEEN 1 AND 18 GRAMS PER LITER, THE QUANTITY OF THE SULPHUR COMPOUNDD BEING SUCH THAT A 10 ML. SAMPLE OF THE BATH WILL REACT WITH AT LEAST 4 ML. OF 0.5 NORMAL IODINE SOLUTION, USINGG STARCH INDICATOR, TO A BLUE ENDPOINT, AND THE AMOUNT OF FREE ACIDITY BEING SUCH AS TO YIELDA PH OF FROM 0.7 TO
 2. 